Cyclone furnaces were developed by The Babcock & Wilcox Company (B&W) in the USA in the 1940's. These Cyclones had the ability to burn high-ash low-fusion temperature coals, which are particularly troublesome in pulverized coal boilers. For a general description of the characteristics of such Cyclone furnaces, the reader is referred to Chapter 14 of Steam/its Generation and Use, 40th Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., .COPYRGT.1992, the text of which is hereby incorporated by reference as though fully set forth herein.
The Cyclone furnace (as schematically shown in FIG. 1) consists of a Cyclone burner connected to a horizontal water-cooled cylinder, the Cyclone barrel. The water circuitry between the Cyclone furnace and the boiler is extremely complex and does not lend itself to being an easily retrofit-type system. Air and crushed coal are introduced through the Cyclone burner into the Cyclone barrel. The larger coal particles are thrust out to the barrel walls by the cyclonic motion of combustion air where they are captured and burned in the molten slag layer that is formed; the finer particles burn in suspension. The mineral matter melts and exits the Cyclone via a tap at the Cyclone throat that leads to a water-filled slag tank (not shown). The combustion gases and remaining ash leave the Cyclone and enter the main furnace.
B&W later helped in the development of the "Secant Firing" technique. This was mainly developed to burn poorer quality German bituminous coals. These coals have lower volatile matter/higher moisture contents which results in associated combustion problems and produced lower Cyclone operating temperatures. To address these operational concerns, B&W helped develop both tangential and secant firing techniques as shown in FIGS. 2 and 3. Finely ground coal is introduced adjacent to the secondary air (tangential firing) or is injected through a series of ports located below the secondary air inlet (secant firing). Based on improved overall Cyclone operational experience, secant firing was chosen as the preferred method.
The Cyclone furnace utilizes centrifugal forces to suspend burning fuel particles, according to their size, in equilibrium against the drag of inwardly directed air flow. The Cyclone furnace has been used with various boiler types: Stirling (SPB), Radiant Boiler (RB) and Universal Pressure (UP) boilers. Due to the centrifugal force created from the Cyclone reentrant throat, the combustion gas flow patterns have been less uniform than that achieved in pulverized coal fired boilers. In one design B&W uses a target wall and/or a screen tube arrangement to straighten the gas flow patterns. FIGS. 4 and 5 illustrate schematically such a Cyclone furnace/screen tube arrangement. These screen tubes were built inside the main boiler and they were a part of the main boiler water circuit. This design of the Cyclone increases the slag and reduces the fly ash entering the main boiler.
When a fuel is burned in a Cyclone boiler the ash content in the fuel is converted to molten slag or it is entrained in the gas as fly ash. It is desirable to reduce the fly ash and increase slag which can be used as construction material, etc. In addition, firing high ash fuels enhances the following concerns: furnace wall slagging, convection pass fouling, and flyash disposal. These negative consequences dictate a need for a lower flyash loading to the main boiler. Slag screen tube arrangements inside the boiler (FIGS. 4 and 5) have been used to help straighten the flue gas flow as well as to improve the ash capture. Although screen tubes are a viable approach in new boiler applications, they cannot be simply applied as a retrofit option. This is due to the complexity of the existing water circuitry between the Cyclone furnace and boiler. Any such retrofit requires a new methodology to incorporate the new Cyclone into the system.
In addition, a solid fuel firing Cyclone furnace cannot be attached to a boiler designed to burn gas and oil. These boilers are not designed for carrying high ash/slag content combustion gases. If a Cyclone furnace alone is added to a boiler, some of the slag/flyash from the Cyclone will enter the main boiler and will be deposited on the heat transfer surfaces. As a result, the boiler heat absorption profiles, steam production, and general boiler operation will be negatively impacted.